A dry film photoresist is a film made of photosensitive material that is widely used in the manufacturing process for a printed circuit board. A conventional dry film photoresist mainly includes a supporting layer, a photosensitive resin layer (namely, a photoresist layer), and a protective layer, where the supporting layer is used as a substrate supporting the photosensitive resin layer and the protective layer is used for protecting the photosensitive resin layer from being damaged during the manufacturing process. Generally, a polyester film having excellent mechanical and optical properties, heat resistance, and dimensional stability, for example, polyethylene terephthalate, is used as a supporting layer. The material, such as polyethylene, polypropylene, or polyester can be used as a protective layer.
The process of applying a dry film photoresist includes peeling off the protective layer before the application of the dry film photoresist on a printed circuit board; and laminating the exposed photosensitive resin layer on a surface of the circuit board (which is generally a copper plate). After application, the supporting layer is located above the photosensitive resin layer. Then, a desired circuit pattern is transferred to the photosensitive resin layer through exposure and development.
It is known in the art that the properties of each layer in a dry film photoresist significantly affect the quality of the photoresist and the property of circuit pattern of a printed circuit board subsequently-formed.
According to ROC (TW) Patent No. 531679, it was found that, although a conventional polyethylene protective layer has flexibility, chemical resistance and good peeling property, gels with a high molecular weight may be generated during polymerization of polyethylene. Such gels may give rise to micro-protrusions (namely, a fish eye) on the polymerized polyethylene film. Due to the presence of such fish eyes, when the photosensitive resin layer is laminated onto a circuit board after the removal of the protective layer, air may enter the interface where the circuit board is laminated to the photosensitive resin layer, and form air holes. Such air holes lead to defects, such as short-circuit, in a circuit board. In order to avoid formation of fish eyes, TW531679 adopts a polyethylene terephthalate film having the following characteristics as a protective layer: 5.0 nm≦SRa≦25 nm, 100 nm≦Spv≦250 nm and 500≦summit density≦10,000, wherein SRa is a surface roughness, Spv is a height difference from the peak to the valley, and the summit density is the number of protrusions with a height of 1 nm or above within a unit area for 4 points (2 μm×2 μm each). According to the disclosure of TW531679, when SRa, Spv and summit density increase, coarse particles form. The adhesion between the photosensitive resin layer and the protective layer increases due to the formation of such coarse particles, and thus fracture of the protective layer when being peeled off tends to occur. In contrast, when SRa, Spv and summit density decrease, the friction coefficient of the protective layer is greatly increased, and thus the problems such as formation of scratches and generation of static electricity may occur during the film process.
According to CN 180600 B, due to the strong electrostatic interaction produced when the protective layer is peeled off, foreign substance may adhere to the film and it is very likely that spark discharge may occur for the photosensitive resin coating containing an organic solvent and cause the dry film photoresist to catch fire.
In view of the foregoing problems, the present disclosure provides a laminated structure of a dry film photoresist including a protective layer having good peeling property, which reduces electrostatic interaction and improves circuit board defects and quality issues resulting from the manufacturing process of the dry film.